Apparatus and method for measuring an induced perturbation to determine a physical condition of the human arterial system

ABSTRACT

A monitor for determining a patient&#39;s physical condition includes a calibration device configured to provide a calibration signal representative of a patient&#39;s physiological parameter. An exciter is positioned over a blood vessel of the patient for inducing a transmitted exciter waveform into the patient. A noninvasive sensor is positioned over the blood vessel, where the noninvasive sensor is configured to sense a hemoparameter and to generate a noninvasive sensor signal representative of the hemoparameter containing a component of a physiological parameter waveform and a component of a received exciter waveform. In this context, a hemoparameter is defined as any physiological parameter related to vessel blood such as pressure, flow, volume, velocity, blood vessel wall motion, blood vessel wall position and other related parameters. A processor is configured to determine a relationship between a property of the received exciter waveform and a property of the physiological parameter. The processor is connected to receive the calibration signal and the noninvasive sensor signal, and the processor is configured to process the calibration signal and the noninvasive sensor signal to determine the physical condition. In an exemplary embodiment, the physical condition is arterial stiffness that represents cardiovascular disease, however, many additional physical conditions can be measured and determined such as arterial elasticity, arterial thickness, arterial wall compliance and other conditions. Also, in an exemplary embodiment, the physiological parameter measured is blood pressure, however, the present invention can also be used to analyze and track other physiological parameters

RELATED APPLICATIONS

This application is a continuation in part of the following patentapplications and incorporates these applications by reference:

Caro, U.S. Ser. No. 08/228,213 filed on Apr. 15, 1994; now U.S. Pat. No.5,590,649 and

Caro, Apparatus and Method for Measuring an Induced Pertubration toDetermine a Physical Condition of the Human Arterial System, U.S.Provisional Application No. 60/605748 filed on Oct. 20, 1995 (AttyDocket No. A-59155-3).

FIELD

The present invention relates to an apparatus and method fornoninvasively providing a determination of a patient's physicalcondition and other clinically important parameters. In particular, theinvention relates to an apparatus and method for measuring an inducedperturbation to determine a physical condition of the human arterialsystem.

BACKGROUND

Blood pressure is the force within the arterial system of an individualthat ensures the flow of blood and delivery of oxygen and nutrients tothe tissue. Prolonged reduction or loss of pressure severely limits theamount of tissue perfusion and could therefore result in damage to oreven death of the tissue. Although some tissues can toleratehypoperfusion for long periods of time, the brain, heart and kidneys arevery sensitive to a reduction in blood flow. The arteries are reliedupon to provide the blood to the brain and other organs. Cardiovasculardisease often results in a hardening of the arteries. When the arteriesare hardened, the arteries are often incapable of providing sufficientblood to the various organs. Cardiovascular disease is a common diseasein humans, and is a leading cause of premature death.

Two of the most important diseases of the cardiovascular system arehypertension, where the patient has elevated blood pressure which causeschronic systemic damage in the circulatory system, and arteriosclerosis,where a change in the physical composition of arteries leads to partialor complete arterial occlusion and restriction of the supply of bloodborne nutrients to critical organs such as the heart and brain.

A great need exists to identify people at risk for these two diseasesearly on so as to allow for the implementation of various diseaseprevention strategies. Furthermore, once the disease is under treatmentit is desirable to have a mechanism for monitoring the disease'sprogression in order to appropriately titrate the treatment.

In hypertension and in arteriosclerosis, advancing disease is associatedwith a progressive change in the physical properties of various arteriesin the cardiovascular system. For example, hypertension is associatedwith the physical hardening of conduit arteries where arteries such asthe carotid artery become increasingly stiff as the disease progresses.Arteriosclerosis commonly results in a hardening of peripheral arterieswhere arteries such as those in the leg become built up with calcifiedor fibrous deposits on the wall of the artery.

Contemporary medical consensus is that a device that can measure thestiffness of the arterial system in some way would have many uses in thediagnosis and treatment of hypertension and arteriosclerosis. As anexample, the observation of peripheral arterial stiffness could indicateearly stages of cardiovascular disease. For patients undergoingtreatment for hypertension using blood pressure lowering drugs, theoptimum drug regime could be determined by monitoring the degree ofinitial arterial stiffness and adjusting therapy to achieve a certainmodification of that stiffness.

If a cheap, noninvasive device was available for monitoring humanarterial stiffness, the device could possibly be used as a screeningtool to determine human candidates who would most benefit from moreexpensive or invasive diagnostic procedures, as well as an adjunct totreatment of the disease. The present invention describes such anapparatus and method for making such a measurement of the physicalproperties of the arterial system.

RELATED ART

A number of studies have demonstrated a relationship between apropagation velocity of the naturally occurring blood pressure wave inthe arterial system and arterial mechanical properties. One studyrelated arterial pulse wave velocity to aortic stiffening. D. Farrar,Aortic Pulse Wave Velocity, Elasticity, and Composition in a NonhumanPrimate Model of Atherosclerosis, Circulation Res., vol. 43, no. 1, p.52 (July 1978). Another study related arterial pulse wave velocity tothe effect of drugs. T. Latson, Effect of Nitroglycerin or AorticImpedance, Diameter, and Pulse Wave Velocity, Circulation Res., vol. 62,no. 5, p. 884 (May 1988). Yet another study related arterial pulse wavevelocity to the effects of obesity and disease. J. Toto-Moukouo, PulseWave Velocity in Patients with Obesity and Hypertension, Am. Heart J.,vol. 112, no. 1, p. 136 (July 1986).

The velocity of the naturally occurring blood pressure wave isinfluenced by the differing properties of the various segments of thearterial system through which it propagates on its way to the periphery.Measurements of the velocity made over significant distances (such asfrom the heart to the radial artery) can be made, but the velocity isthen dependent on some average of the differing arterial properties ofthe arteries along the propagation path. Measurement of the velocity ofthe naturally occurring pulse over short distances such as over a smallsegment of the radial artery, are error prone due to the long durationof the pulse with respect to the time taken for its propagation. Forthese reasons it is difficult to use the natural pulse wave velocity asa reliable indicator of arterial physical condition.

Techniques for diagnosing and treating cardiovascular disease aredescribed in U.S. Pat. Nos. 5,054,493; 5,211,177; and 5,316,004, whereanalysis of the waveform of the blood pressure pulse shape, detected ata peripheral artery such as the radial artery, is used to determine thecompliance of an idealized model of the arterial system. That systemiccompliance might have some useful relationship to systemic disease inthe patient. Unlike the invention described here, in which measurementsof arterial properties are made that apply to a specific arterialsegment, this analysis involves determination of a value of compliancethat is a form of averaged compliance representative of the entiresystem. Furthermore, the large amplitude of the natural pressure pulseallows various nonlinearities in the propagation equations to becomeimportant, thus rendering an analysis complex.

Another technique is described in U.S. Pat. No. 5,152,297, wherearterial diameter and pressure are simultaneously measured and theelastic properties of the artery are measured therefrom.

In a study described by M. Anliker, Dispersion and Attenuation of SmallArtificial Pressure Waves in the Canine Aorta, Circulation Res., vol.23, p. 539 (October 1968), high frequency small-signal pressureperturbations were introduced into the arteries of dogs and the velocityof propagation was measured using an invasive pressure detector placedin the artery. The possibility of using these measurements to determinevarious physical properties of the arterial system is discussed.However, Anliker employs an invasive technique that has obviousdrawbacks including infection and healing consequences.

OBJECTS AND SUMMARY

The present invention relates to an apparatus and method fornoninvasively providing a determination of a patient's physicalcondition and other clinically important parameters. In particular, theinvention relates to an apparatus and method for measuring an inducedperturbation to determine a physical condition of the human arterialsystem.

An object of the invention is to induce a perturbation into a patient'sblood or blood vessel and to noninvasively measure the perturbation todetermine the patient's physical condition.

A related object is to filter the noninvasive sensor signal intocomponents including a natural component, an induced component and anoise component, and to process the components to determine thepatient's physical condition.

A further related object is to determine a relationship between aproperty of an induced perturbation and a property of a physiologicalparameter.

In the parent patent applications of Caro et al, techniques aredescribed for the measurement of physiological parameters such as bloodpressure, arterial elasticity, cardiac output, and other parameters. Aspart of those techniques, a procedure is described to determine arelationship between a patient's blood pressure P and the velocity ofpropagation Vel of an induced high frequency pressure perturbation alongthe artery. This relationship can have various forms and can bedescribed generally as velocity equation:

    Vel=f(P)

where f(P) is a function of pressure. An important step in the Carotechniques is the determination of the nature of the function f over arange of pressure in a given patient. In the Caro techniques, therelationship of the velocity equation was used to determine pressurefollowing a measurement of velocity Vel. The present invention employsthe information contained in the form of the function f to determineinformation relating to the physical condition of the arterial system.

The information regarding the physical condition of the arterial systemcan then be used to generate an output that can be used by health carepersonnel in the diagnosis and management of diseases such ascardiovascular disease.

A monitor for determining a patient's physical condition includes acalibration device configured to provide a calibration signalrepresentative of a patient's physiological parameter. An exciter ispositioned over a blood vessel of the patient for inducing a transmittedexciter waveform into the patient. A noninvasive sensor is positionedover the blood vessel, where the noninvasive sensor is configured tosense a hemoparameter and to generate a noninvasive sensor signalrepresentative of the hemoparameter containing a component of aphysiological parameter waveform and a component of a received exciterwaveform. In this context, a hemoparameter is defined as anyphysiological parameter related to vessel blood such as pressure, flow,volume, velocity, blood vessel wall motion, blood vessel wall positionand other related parameters. A processor is configured to determine arelationship between a property of the received exciter waveform and aproperty of the physiological parameter. The processor is connected toreceive the calibration signal and the noninvasive sensor signal, andthe processor is configured to process the calibration signal and thenoninvasive sensor signal to determine a physical property of thepatient in order to provide a condition signal related to the patient'sphysical condition.

In an exemplary embodiment, the physical property is arterial stiffnessthat represents a physical condition such as cardiovascular disease,however, many additional physical property can be measured anddetermined such as arterial elasticity, arterial thickness, arterialwall compliance and other properties that can provide information fordetermining the patient's physical condition.

Also, in an exemplary embodiment, the physiological parameter measuredis blood pressure, however, the present invention can also be used toanalyze and track other physiological parameters such as vascular wallcompliance, strength of ventricular contractions, vascular resistance,fluid volume, cardiac output, myocardial contractility and other relatedparameters.

BRIEF DESCRIPTION OF THE FIGURES

Additional advantages of the invention will become apparent upon readingthe following detailed description and upon reference to the drawings,in which:

FIG. 1 depicts the present invention attached to a patient;

FIG. 2 depicts an exciter attached to a patient;

FIG. 3 depicts a noninvasive sensor attached to a patient;

FIG. 4a depicts a blood pressure waveform;

FIG. 4b depicts a blood pressure waveform with an exciter waveformsuperimposed thereon;

FIG. 5 depicts a schematic diagram of the present invention;

FIG. 6, 6a-b depict a processing flow chart according to one embodimentof the invention;

FIGS. 7a-c are graphical illustrations of the filter procedures of thepresent invention;

FIGS. 8a-c are graphical illustrations showing the relationships betweenthe exciter waveform and blood pressure;

FIGS. 8, 9a-b depict a processing flow chart according to anotherembodiment of the invention;

FIGS. 10a-b depict a processing flow chart according to anotherembodiment of the invention;

FIG. 11 depicts an exciter and noninvasive sensor attached to a patient;

FIG. 12 depicts a pressure redetermination apparatus according to anembodiment of the invention;

FIG. 13 is a graphical illustration showing the relationship betweentypical artery radius and pressure; and

FIG. 14 is a graphical illustration showing a typical exciter waveformvelocity versus blood pressure.

    ______________________________________                                        GLOSSARY                                                                      ______________________________________                                        P.sub.D  diastolic blood pressure                                             P.sub.DO diastolic blood pressure at calibration                              P.sub.S  systolic blood pressure                                              P.sub.p  pulse pressure                                                       P.sub.w  exciter waveform pressure                                            V.sub.d  received exciter waveform                                            V.sub.W  signal exciter waveform                                              V.sub.n  noise waveform                                                       V.sub.e  exciter sensor signal (transmitted exciter waveform)                 V.sub.P  detected pulsatile voltage                                           Φ.sub.W                                                                            exciter signal phase                                                 ΦW.sub.D                                                                           exciter signal phase at diastole                                     Vel(t)   exciter signal velocity                                              Vel.sub.D                                                                              exciter signal velocity at diastole                                  Vel.sub.S                                                                              exciter signal velocity at systole                                   ______________________________________                                    

DETAILED DESCRIPTION

The present invention relates to an apparatus and method fornoninvasively providing a determination of a patient's physicalcondition and other clinically important parameters. In particular, theinvention relates to an apparatus and method for measuring an inducedperturbation to determine a physical condition of the human arterialsystem.

A preferred embodiment concentrates on the physical property of arterialstiffness that represents a physical condition such as cardiovasculardisease, however, many additional physical properties can be measuredand determined such as arterial elasticity, arterial thickness, arterialwall compliance and other properties that can provide information fordetermining the patient's physical condition. Also, the preferredembodiment concentrates on the physiological parameter of bloodpressure, however, many additional physiological parameters can bemeasured with the present invention including vascular wall compliance,ventricular contractions, vascular resistance, fluid volume, cardiacoutput, myocardial contractility and other related parameters. Thoseskilled in the art will appreciate that various changes andmodifications can be made to the preferred embodiment while remainingwithin the scope of the present invention. As used herein, the termcontinuous means that the physiological parameter of interest isdetermined over a period of time, such as during the course of surgery.The implementation of portions of the invention in a digital computer isperformed by sampling various input signals and performing the describedprocedures on a set of samples. Hence, a periodic determination of thephysiological parameter of interest is within the definition of the termcontinuous.

FIG. 1 illustrates the components and configuration of the preferredembodiment. Oscillometric cuff 110 is connected to processor 100 viawire 106, and cuff 110 is responsive to processor 100 during an initialcalibration step. Oscillometric cuff operation, which is known in theart, involves an automated procedure for obtaining a blood pressuresignal. The general procedure is given for clarity but is not crucial tothe invention.

First, an occlusive cuff is pressurized around the patient's upper armto abate the blood flow. Then, as the pressure is slowly reduced, atransducer senses when the blood flow begins and this pressure isrecorded as the systolic pressure. As the pressure is further reduced,the transducer similarly detects the pressure when full blood flow isrestored and this pressure is recorded as the diastolic pressure. Thesignals representing pressure are delivered, via wire 106, to processor100 for storage. An alternative blood pressure measurement techniquesuch as manual or automated sphygmomanometry using Korotkoff sounds or"return to flow" techniques, could also be used. A manual measurementcan be provided, for example, using a keypad. Whatever measurementtechnique is used, a calibration device provides a calibration signalrepresentative of the patient's physiological parameter. In thisrespect, the calibration device is broadly defined to include automatedor manual measurements.

FIG. 1 shows an exciter 202 attached to the patient's forearm above theradial artery. The exciter 202 is a device for inducing a perturbationof the patient's body tissue, and is controlled by the processor 100 viatube 107.

FIG. 2 shows a cross section of the exciter and its components. Theexciter 202 is an inflatable bag attached to the processor via air tube107. It is fixed in place near an accessible artery 220 by holddowndevice 204 which can be a buckle, adhesive strap or other device. Thereis also an exciter sensor 203 disposed within the exciter to generate areference signal indicative of the perturbation source waveform, and todeliver the signal to the processor via wire 108. This signal is used asa reference signal by the processor (explained below).

As mentioned above, processor 100 is attached to the exciter via tube107. The processor 100 controls the pressure in exciter 202 with atransducer and diaphragm. A transducer is a device that transforms anelectrical signal to physical movement, and a diaphragm is a flexiblematerial attached to the transducer for amplifying the movement. Anexample of this combination is a loudspeaker. The diaphragm forms partof an airtight enclosure connected to air tube 107 and an input toinitialize the pressure. It will be clear to one skilled in the art thatthe transducer and air tube 107 and exciter 202 can be miniaturized andcombined into a single exciter element capable of acting as a vibratingair filled bag connected to the processor by an electrical drive signalalone, in the case that a source of substantially constant pressure suchas a spring is included in the exciter, or by an electrical drive signaland connection to a source of substantially constant pressure for thebag.

In operation, the pressure is initially established via theinitialization input and then the pressure is varied by an electricalsignal delivered to the transducer; the diaphragm produces pressurevariations in the tube in response to the transducer movement. Theresult is that the processor, by delivering an oscillating electricalsignal to the transducer, causes oscillating exciter pressure. Theexciter responds by perturbing the patient's tissue and inducing atransmitted exciter waveform into the patient.

The perturbation excites the tissue 221 and blood vessel 220 below theexciter and causes the transmitted exciter waveform to radiate withinthe patient's body, at least a portion of which travels along the bloodfilled vessel. The excitation waveform can be sinusoidal, square,triangular, or of any suitable shape. Experiments conducted to determinea range of satisfactory perturbation frequencies found that the range of20-1000Hz works well. It is anticipated that frequencies of lesser than20Hz and greater than 1000 Hz will also work well, and it is intendedthat this specification cover all frequencies insofar as the presentinvention is novel.

FIG. 1 further shows a noninvasive sensor 210 placed at a distance fromthe exciter on the patient's wrist. The noninvasive sensor is connectedto the processor 100 via wire 109.

FIG. 3 shows a cut-away view of the noninvasive sensor 210 placed overthe same radial artery 220 as the exciter. The sensor 210 is fixed inplace near the artery 220 by holddown device 211 which can be a buckle,adhesive strap or other device. The holddown device 211 also includes abaffle 212 to reduce noise, where the baffle is a pneumatic bagpressurized to hold the sensor 210 at a constant pressure against thepatient, for example at a pressure of 10 mm Hg. Alternately, baffle 212can be any suitable device such as a spring or foam pad.

The noninvasive sensor 210 is responsive to at least one hemoparameterof the patient and generates a signal in response thereto. In thiscontext, a hemoparameter is defined as any physiological parameterrelated to vessel blood such as pressure, flow, volume, velocity, bloodvessel wall motion, blood vessel wall position and other relatedparameters. In the preferred embodiment a piezoelectric sensor is usedto sense arterial wall displacement, which is directly influenced byblood pressure.

As is shown, the sensor is positioned over the radial artery 220 and itis responsive to pressure variations therein; as the pressure increases,the piezoelectric material deforms and generates a signal correspondingto the deformation. The signal is delivered to the processor 100 viawire 109.

FIG. 1 also shows the processor 100 that has a control panel forcommunicating information with the user. A power switch 101 is forturning the unit on. A waveform output monitor 102 displays thecontinuous blood pressure waveform for medical personnel to see. Thiswaveform is scaled to the pressures determined by the processor, andoutput to the monitor. A digital display 103 informs the user of thecurrent blood pressure; there is a systolic over diastolic and meanpressure shown. A calibrate button 104 permits the user to calibrate theprocessor at any time, by pressing the button. The calibration display105 shows the user the blood pressure at the most recent calibration,and also the elapsed time since calibration. A physical conditiondisplay 120 is also provided to show the patient's physical condition.The patient's physical condition is determined based on proceduresdescribed below, but for example can include artery radius and arterialdistensibility. The processor maintains a record of all transactionsthat occur during patient monitoring including calibration bloodpressure, calibration times, continuous blood pressure and otherparameters, and it is anticipated that additional information can bestored by the processor and displayed on the control panel.

Turning to the noninvasive sensor signal, in addition to a naturalhemoparameter, the noninvasive sensor signal contains a componentindicative of the exciter waveform traveling through the patient.Although the exciter component is designed to be small in comparison tothe natural hemoparameter, it contains valuable information. Therefore,the processor is used to separate the exciter waveform from the naturalhemoparameter, and to quantify the respective components to determinethe patient's blood pressure.

FIG. 4a shows a natural blood pressure waveform where the minimumrepresents the diastolic pressure and the maximum represents thesystolic pressure. This waveform has a mean arterial pressure (MAP) thatis a convenient reference for purposes of determining the DC offset ofthe waveform. Example pressure values are 90 mm Hg diastolic and 120 mmHg systolic respectively with a MAP DC offset of 80 mm Hg.

FIG. 4b shows an operational illustration of the arterial waveform; anexciter waveform superimposed on a natural blood pressure waveform. Theexciter induces the exciter waveform into the arterial blood at a firstlocation and the exciter waveform becomes superimposed on the naturalwaveform. Since the exciter waveform is small compared to the patient'snatural waveform, the natural waveform dominates as shown in FIG. 4b. Asmentioned above, the noninvasive sensor signal contains informationregarding both the natural waveform and the exciter waveform. Theprocessor 100 is designed to separate the constituent components of thenoninvasive sensor signal to continuously determine the patient's bloodpressure, as is discussed below.

FIG. 5 depicts a schematic diagram of the preferred embodiment. There isan oscillometric cuff controller 121 for controlling the oscillometriccuff and determining the readings therefrom to generate a signalrepresenting the patient's blood pressure. There is an induced wavefrequency generator 131 coupled to a pressure transducer 133 thattransforms an electrical input to a pressure output. The transduceroutput is connected to the exciter 202 and controls the exciter'soscillations for inducing the exciter waveform into the patient'sarterial blood.

The output of the exciter sensor 203 is fed to a band pass filter 134.This filter 134 separates the high frequency signal responsive to thetransducer pressure and delivers the resulting signal to RMS meter 135and to lock-in amplifier 143 reference input. In the preferredembodiment, the RMS meter output is sampled at a rate of 200 samples persecond with a 14 bit resolution and delivered to computer 150. It isanticipated that the sampling rate and resolution can be varied withgood results.

The output of the noninvasive sensor is fed to a charge amplifier 140that delivers a resulting signal to a low pass filter 141 and a bandpass filter 142. These filters separate the noninvasive sensor signalinto two constituent components representing an uncalibrated naturalblood pressure wave and a received exciter waveform respectively. Thelow pass filter output is sampled at a rate of 200 samples per secondwith 14 bit resolution and delivered to computer 150, and the band passfilter output is delivered to the lock-in amplifier 143 signal input.

The lock-in amplifier 143 receives inputs from band pass filter 134 asreference and band pass filter 142 as signal, which are the excitersensor signal (transmitted exciter waveform) and noninvasive sensorexciter signal (received exciter waveform) respectively. The lock-inamplifier uses a technique known as phase sensitive detection to singleout the component of the noninvasive sensor exciter signal at a specificreference frequency and phase, which is that of the exciter sensorsignal. The amplifier 143 produces an internal, constant-amplitude sinewave that is the same frequency as the reference input and locked inphase with the reference input. This sine wave is then multiplied by thenoninvasive sensor exciter signal and low-pass filtered to yield asignal proportional to the amplitude of the noninvasive sensor signalmultiplied by the cosine of the phase difference between the noninvasiveexciter signal and the reference. This is known as the in-phase or realoutput.

The amplifier 143 also produces an internal reference sine wave that is90 degrees out-of-phase with the reference input. This sine wave ismultiplied by the received exciter signal and low-pass filtered to yielda signal proportional to the amplitude of the noninvasive sensor signalmultiplied by the sine of the phase difference between the noninvasivesensor exciter signal and the reference. This is known as quadrature orimaginary output. The amplifier 143 then provides the computer 150 withinformation regarding the real and imaginary components of the receivedexciter signal as referenced to the phase of the transmitted excitersignal. Alternately, the amplifier can provide components representingthe magnitude and phase of the received exciter signal. In the preferredembodiment, the amplifier output is sampled at a rate of 200 samples persecond with a 14 bit resolution. It is anticipated that the samplingrate and resolution can be varied with good results.

The computer 150 receives input from the oscillometric cuff controller121, RMS meter 135, low pass filter 141 and lock-in amplifier 150. Thecomputer 150 also receives input from the user interface panel 160 andis responsible for updating control panel display information. Thecomputer 150 executes procedures for further separating constituentcomponents from the noninvasive sensor signal and attenuating thenoninvasive sensor noise component as shown in FIG. 6.

While the processing system described in the embodiments involves theuse of a lock-in amplifier 143, it will be clear to those personsskilled in the art that similar results can be achieved by frequencydomain processing. For example, a Fourier transform can be performed onthe various signals to be analyzed, and processing in the frequencydomain can be further performed that is analogous to the describedprocessing by the lock-in amplifier in the time domain. The variousfiltering steps described above can be advantageously performed in thefrequency domain. Processing steps in the frequency domain areconsidered as falling within the general category of the analysis of thetransfer function between the exciter sensor waveform and thenoninvasive sensor waveform and are intended to be covered by theclaims. The variety of techniques that are used in the art for thecalculation of transfer functions are also applicable to this analysis.

PROCESS EXCITER WAVEFORM VELOCITY TO DETERMINE OFFSET SCALING ANDEXCITER WAVEFORM AMPLITUDE TO DETERMINE GAIN SCALING

FIG. 6 is a processing flowchart that represents the operation of theFIG. 5 computer 150. The operation begins at step 702 with the receiptof an initial calibration measurement; noninvasive sensor signal andexciter sensor signal. Step 704 chooses the blood pressure waveformsegment for pulse reference, which is important for continuity ofmeasurement from pulse to pulse and for consistency over periods of timebetween calibration measurements. In this embodiment, the diastolicpressure (low-point) is chosen for purposes of simplicity, but any pointof the waveform can be chosen such as the systolic pressure or meanarterial pressure (MAP). The choice of the segment will relate to the DCoffset discussed below.

Step 706 is a filter step where the noninvasive sensor (received)exciter waveform is separated into signal and noise components. Thenoise components may have many sources, one of which is a signal derivedfrom the exciter that travels to the noninvasive sensor by an alternatepath, other than that along the artery taken by the signal of interest.Examples include bones conducting the exciter waveform and the surfacetissue such as the skin conducting the exciter waveform. Additionalsources of noise result from patient movement. Examples includevoluntary patient motion as well as involuntary motion such as movementof the patient's limbs by a physician during surgery.

FIGS. 7a-c illustrate the principles of the received exciter signalfiltering. During a natural pulse, the received exciter waveform V_(d)is represented by a collection of points that are generated in thecomplex plane by the real and imaginary outputs of the lock-in amplifier143 which is monitoring the noninvasive sensor signal. FIG. 7arepresents the received exciter waveform V_(d) in the absence of noise.In the absence of noise, V_(d) is the same as vector V_(w) (t) which hasa magnitude and phase corresponding to the received exciter signal.During a pulse, the magnitude of V_(w) (t) remains constant, but theangle periodically oscillates from a first angle representing a lesserpressure to a second angle representing a greater pressure. Note that inthe absence of noise, the arc has a center at the origin.

FIG. 7b represents the received exciter waveform V_(d) in the presenceof noise, which is indicated by vector V_(n). Vector V_(d) has amagnitude and phase according to the noninvasive sensor exciter waveformplus noise. As can be seen in FIGS. 7b-c, vector V_(d) (t) defines acollection of points forming an arc having a common point V_(c)equidistant from each of the collection of points. The vector V_(w) (t)from V_(c) to the arc corresponds to the true magnitude and phase of thenoninvasive signal exciter waveform. The vector V_(n) represents noiseand, once identified, can be removed from the noninvasive sensorwaveform. The filter step removes the V_(n) noise component and passesthe V_(w) (t) signal exciter component on to step 708.

In the above discussion, it was assumed for illustrative purposes thatthe magnitude of V_(w) (t) remains constant over the time of a pulse. Insome cases the attenuation of the exciter waveform as it propagatesalong the artery is pressure dependent, and in those cases the magnitudeof V_(w) (t) can vary during the pulse in a way that is correlated topressure. Under such circumstances the shape of the figure traced out inthe complex plane by the vector V_(d) will deviate from a perfect circlesegment. A typical shape is that of a spiral with a form that can bepredicted theoretically. The functioning of this filter step under suchcircumstances is conceptually similar to that described above, exceptthat the elimination of the noise vector V_(n) must involve location ofthe origin of a spiral rather than of the center of a circle.

Step 708 determines if the pulse is valid. To do this, the processorchecks the constituent components of the noninvasive sensor signal toinsure that the components are within acceptable clinical norms of thepatient. For example, the processor can determine whether the new pulseis similar to the prior pulse, and if so, the new pulse is valid.

Step 720 processes the signal exciter waveform V_(w) (t) to determinethe DC offset. For convenience the diastole is used as the offset value,but any part of the waveform can be used. The processor determines theoffset when the vector V_(w) (t) reaches its lowest phase angle (i.e.,the maximum clockwise angle of FIG. 7a); this is the diastole phaseangle Φ_(w) (dias). A calibration diastolic measurement is stored by theprocessor at calibration as P_(D0). Also stored by the processor is arelationship denoting the relationship between the velocity of anexciter wave and blood pressure. This relationship is determined byreference to a sample of patients and is continuously updated byreference to the particular patient after each calibration measurement.FIG. 8a-c are graphical illustrations showing clinically determinedrelationships between the exciter waveform and blood pressure. FIG. 8brepresents the relationship between phase and pressure at a frequency of150Hz; other frequencies have relationships that are vertically offsetfrom the line shown. The pressure-velocity relationship represents thestorage of this graphical information either in a data table or by ananalytical equation.

Step 721 determines the predicted diastolic pressure from theinformation in Step 720. The processor continuously determines thechange in diastole from one pulse to the next by referencing theposition of the signal exciter vector V_(w) (t), at Φ_(w) (dias), withrespect to the stored pressure-velocity relationship. Moreover, thepressure-velocity relationship is continuously updated based oncalibration measurement information gained from past calibrations of thepatient.

A set of established relationships is used to develop and interpretinformation in the table and to relate the information to the sensorsignal components. First, a known relationship exists between bloodpressure and exciter waveform velocity. Also, at a given frequency manyother relationships are known: a relationship exists between velocityand wavelength, the greater the velocity the longer the wavelength; anda relationship exists between wavelength and phase, a change inwavelength will result in a proportional change in phase. Hence, arelationship exists between blood pressure and phase, and a change inblood pressure will result in a proportional change in phase. This isthe basis for the offset prediction.

With the stored calibration measurement plus the change in diastole, thenew DC offset diastolic pressure is predicted P_(D) (pred). Thisprediction is made based on the diastolic pressure at calibration P_(D0)plus the quotient of the phase difference between calibration Φw_(D0)and the present time Φw(dias) and the pressure-velocity relationshipstored in processor memory as rate of change of exciter waveform phaseto pressure d(Φw_(D))/dP. ##EQU1##

Step 722 displays the predicted diastolic pressure.

Step 730 determines the noninvasive sensor exciter waveform phase andvelocity. This determination is made based on the comparison of thenoninvasive sensor exciter waveform with the exciter sensor waveform.

Step 731 determines the noninvasive sensor exciter waveform amplitudefrom the noninvasive sensor signal.

Step 732 determines the exciter waveform pressure P_(w) by multiplyingthe exciter sensor waveform magnitude V_(e) by the ratio of thecalibrated exciter waveform pressure P_(w) (cal) to the calibratedexciter sensor waveform magnitude V_(e) (cal). ##EQU2## In situationswhere a significant pressure variation can be observed in theattenuation of the exciter waveform as it propagates from exciter todetector, an additional multiplicative pressure dependent correctionfactor must be included in equation 2.

Step 734 determines if the calibration values are still valid. Thisdetermination can be based on many factors including the time since thelast calibration, that the linearity of the pressure-velocityrelationship is outside of a reliable range, determination by medicalpersonnel that a new calibration is desired or other factors. As anexample of these factors, the preferred embodiment provides usersettable calibration times of 2, 5, 15, 30, 60 and 120 minutes, andcould easily provide more. Moreover, the curve upon which the pressureis determined is piecewise linear with some degree of overallnonlinearity. If the processor 100 determines that the data isunreliable because the linear region is exceeded, the processor willinitiate a calibration step. Finally, if the operator desires acalibration step, a button 104 is provided on the processor 100 forinitiating calibration manually.

Step 736 predicts a new pulse pressure P_(p) (pred) by multiplying theexciter waveform pressure P_(w) by the ratio of the detected pulsatilevoltage V_(p) to the detected exciter waveform magnitude V_(w). ##EQU3##

This prediction uses the noninvasive sensor exciter waveform todetermine the pressure difference between the diastole and systole ofthe natural blood pressure waveform. For example, if a noninvasivesensor exciter magnitude V_(w) of 0.3 V relates to a pressure variationP_(w) of 1 mm Hg and the noninvasive sensor waveform V_(p) varies from-6 V to +6 V, then the noninvasive sensor waveform represents a pulsepressure excursion P_(p) (pred) of 40 mm Hg.

Step 760 predicts a new systolic pressure P_(s) (pred) by adding thepredicted diastolic P_(D) (pred) and pulse pressures P_(p) (pred).

    P.sub.s (pred)=P.sub.D (pred)+P.sub.p (pred)               (4)

In the above example if the diastole PD(pred) is 80 mm Hg (DC offset)and the pulse P_(p) (pred) represents a difference of 40 mm Hg then thenew systolic P_(s) (pred) is 120 mm Hg. Then the new systolic pressureis displayed.

For display purposes the values determined for P_(S) (pred) and P_(D)(pred) can be displayed numerically. Similarly, the output waveform fordisplay 102 can be displayed by scaling the noninvasive sensor naturalblood pressure waveform prior to output using gain and offset scalingfactors so that the output waveform has amplitude, P_(P) (pred), and DCoffset, P_(D) (pred), equal to those predicted in the above process. Thescaled output waveform signal can also be output to other instrumentssuch as monitors, computers, processors and displays to be used fordisplay, analysis or computational input.

Step 750 is taken when step 734 determines that the prior calibration isno longer reliable as described above. A calibration step activates theoscillometric cuff 201 and determines the patient's blood pressure, asdescribed above. The processor 100 uses the calibration measurements tostore updated pressure and waveform information relating to the DCoffset, blood pressure waveform and exciter waveform. The updatedvariables include calibration pulse pressure P_(p) (cal), calibrationexciter sensor waveform magnitude V,(cal), diastolic pressure P_(D0),diastolic exciter waveform phase Φw_(D0), the rate of change of exciterwaveform phase to pressure d(Φw_(D))/dP and calibration exciter waveformpressure P_(w) (cal). ##EQU4## PROCESS EXCITER WAVEFORM VELOCITY TODETERMINE OFFSET SCALING AND GAIN SCALING

FIGS. 9a-b represent a modification to the previous embodiment. Theinitial processing steps 702, 704, 706, 708, 730 and 731 represented inthe flow chart of FIG. 9 are substantially similar to those described inthe previous embodiment depicted in FIG. 6. In step 730, exciterwaveform velocity Vel(t) and the actual phase delay of the exciterwaveform Φ(t) are related by the equation:

    Φ(t)=Φ.sub.0 -2πdf/Vel(t)                       (6)

where frequency f and distance d between exciter and noninvasive sensorare known. The constant Φ₀ is determined in advance either analyticallyor empirically, and is dependent on the details of the geometry of theapparatus.

Measurement of Φ(t) is generally made modulo 2π, and so the measuredphase Φ_(m) (t) is related to actual phase delay by the equation:

    Φ.sub.m (t)=Φ(t)+2nπ                            (7)

where n is an integer also known as the cycle-number, typically in therange of 0-10. While correct deduction of propagation velocity requiresa correct choice of n, a correct prediction of pressure using apressure-velocity relation does not, so long as the same value of n isused in determining Φ(t) and in determining the pressure-velocityrelationship. In such a case, velocity should be considered as apseudo-velocity rather than an actual measure of exciter waveformpropagation speed.

In step 730, therefore, use of the Φ(t) equations allows determinationof the velocity, or pseudo-velocity, Vel(t) as a function of time. Instep 801, the values of velocity at the systolic and diastolic points ofthe cardiac cycle are determined as Vel_(S) and Vel_(D). Thesecorrespond to the points of minimum and maximum phase delay or to thepoints of maximum and minimum amplitude of the naturally occurring bloodpressure wave detected by the noninvasive sensor. Use of thepressure-velocity relationship stored in the processor is then made totransform the values of velocity at systolic and diastolic points intime to values of pressure. In step 803 the diastolic pressure isdetermined using the equation:

    P.sub.D (pred)=P.sub.D0 +(Vel.sub.D -Vel.sub.D0)/(dVel/dP) (8)

Step 804 is performed to determine the predicted systolic pressureaccording to the relationship:

    P.sub.S (pred)=P.sub.D (pred)+(Vel.sub.S -Vel.sub.D)/(dVel/dP) (9)

In this illustration the values of P_(S) and P_(D) are used to determinethe pressure waveform. Similarly, other pairs of values, such as meanpressure and pulse pressure can also be used, and appropriatepermutations of the predicted pressure. equations are anticipated bythis description.

In step 805 the calculated pressures are displayed as numbers, with atypical display comprising display of mean, systolic and diastolicvalues of the pressure waveform in digital form, together with theobserved pulse rate. The values of P_(D) (pred) and P_(S) (pred) areused to determine appropriate gain and DC offset scaling parameters bywhich the naturally occurring blood pressure waveform detected by thenoninvasive sensor is scaled prior to output in step 806 as a timevarying waveform, shown as 102 in FIG. 1.

As in the embodiment depicted in FIG. 6, step 750 involves a calibrationstep initiated when step 734 determines that the prior calibration is nolonger reliable. During the performance of step 750 thepressure-velocity relationship is determined and stored in the processorin the form of a table or of an analytical relationship. During thisprocess, it may be desirable to stop the output portion of the processas shown in step 752 and display a different signal, such as a blankscreen, a dashed line display, a blinking display, a square wave, orsome other distinguishable signal of calibration such as an audibletone. This step is represented as step 808 in FIG. 9.

PROCESS EXCITER WAVEFORM VELOCITY TO DETERMINE OUTPUT BLOOD PRESSUREWAVEFORM

In both of the previous two embodiments, values of gain Pp(pred) andoffset P_(D) (pred) are determined and used to scale the noninvasivesensor natural blood pressure waveform to provide a time varying outputwaveform representative of the patient's blood pressure. In thisembodiment, the natural blood pressure waveform monitored by thenoninvasive sensor is not used in the determination of the output bloodpressure waveform. As in the previous embodiment, use is made of therelationship between velocity of the exciter waveform and the bloodpressure of the patient to determine the blood pressure. Rather thanmaking such a pressure determination only at the diastolic and systolicpoints of the cardiac cycle, exciter waveform velocity is measured manytimes during a cardiac cycle (typically 50-200 times per second) and theresultant determinations of pressure are used to construct the outputtime varying blood pressure waveform. This process is described belowwith reference to FIG. 10.

In this embodiment, the natural blood pressure waveform is not scaled.Therefore, there is no need to separate the data into pulse segments asin step 704 of FIG. 6. This feature greatly simplifies the computationaltask. An additional advantage of this technique is that all of theinformation used in the analysis process is encoded in the exciterwaveform, which is typically at a high frequency compared with that ofboth the natural blood pressure waveform and that of any artifactsignals introduced by patient motion or respiration. Since all of theselower frequency signals can be removed by electronic filtering, thistechnique is extremely immune to motion induced artifact and similarsources of interference that might otherwise introduce errors into themeasurement.

With the exception of this step, the initial processing steps 702,706,731 and 730 are substantially similar to those of previously describedembodiments. The amplitude and phase of the exciter waveform determinedin step 731 are continuous functions of time. The exciter waveform phaseis converted to exciter waveform velocity as described previously, whichis also a continuous function of time.

Using a relationship between pressure and velocity, determined during orsubsequent to the initial calibration and periodically redetermined, thetime dependent velocity function Vel(t) is readily transformed to a timedependent pressure function P(t). This transformation is represented bystep 802. In a typical case, the pressure-velocity relationship might beas follows:

    Vel(t)=a+bP(t)                                             (10)

where the constants a and b were determined during step 750. In thatcase the velocity equation (10) can be used to perform thetransformation of step 802.

Following a variety of checking steps, described below, that ensure thetransformation used in 802 was correct, the minimum and maximum pointsof P(t) are determined for each cardiac cycle and displayed as P_(D)(pred) and P_(S) (pred) in step 805. Then, in step 806, the entire timedependent waveform is displayed as waveform 102.

DETERMINATION OF THE PRESSURE-VELOCITY RELATIONSHIP

In each of the embodiments described thus far, an important stepinvolves the conversion of a measured phase to a deduced exciterwaveform velocity, and conversion of that value to a pressure. In thecase of the flow chart of FIG. 6, this process is integral to thecalculation of the DC offset pressure P_(D0). In the case of theembodiment described in FIG. 9, this process is integral todetermination of P_(S) and P_(D). In the case of the embodimentdescribed in FIG. 10, the process is integral to the determination ofpressure at each point in time for which an output value is to bedisplayed as part of a "continuous" pressure waveform display.

The relationship between pressure and velocity is dependent on manyfactors including the elastic properties of the artery along which theexciter waveform travels. This relationship varies considerably betweenpatients, and must therefore be determined on a patient by patientbasis, although a starting relationship derived from a pool of patientscan be used. This determination occurs during step 750 in each of theembodiments described in FIGS. 6, 9, and 10, and the relationship isstored in the processor in either a tabular form, or as an analyticalrelationship. In step 734 in FIGS. 6, 9 and 10, a variety of parametersare examined to determine whether the system calibration continues to beacceptable. As part of that process, it is determined whether theexisting pressure-velocity relationship continues to be valid. If not, arecalibration can be initiated.

In most patients there is a monotonically increasing relationshipbetween velocity of propagation of the induced perturbative pressureexcitation along the arterial system and pressure. Over a certain rangethis relationship can be approximated as linear. In some cases a morecomplicated functional relationship between pressure and velocity mayneed to be used. In general the relationship can be well described by anequation of order 1 or 2 and the collection of a series of (pressure,velocity) value pairs and the use of a fitting process allows thedetermination of an appropriate relationship between pressure andvelocity. In some cases, use of a predetermined general relationshipwith coefficients dependent on patient parameters such as weight,height, heart rate or age is possible.

One technique for pressure-velocity relation determination involvesdetermination of pressure at diastolic, mean and systolic points at thesubstantially simultaneous time that velocity measurements are made atthe same three points in the cardiac cycle. These three pairs of pointscan then be fit to determine a pressure-velocity relationship.

In one embodiment of the pressure-velocity relationship determinationprocess, an occlusive cuff measurement is made on the contralateral arm(opposite arm) to that upon which the perturbation and detection processare occurring. It is then possible to perform a conventional cuff basedmeasurement of blood pressure in one arm, yielding measurement of mean,systolic and diastolic pressures, substantially simultaneously withmeasurements of mean, systolic and diastolic velocities in the oppositearm. So long as it has been ascertained in advance that the patient hassubstantially similar pressures in both arms and that the two arms areeither maintained at constant heights or correction is made forhydrostatic pressure differences between limbs, then it is valid to usethe pressure in one arm as a proxy for pressure in the other. In thisway it is possible to obtain three pairs of pressure and velocitymeasurements taken during a single time interval. A curve fittingprocess can be used to determine a pressure-velocity relationship thatbest describes this data and that relationship can be used as the basisfor future prediction of pressure from measured velocity. In general ithas been found that the use of a linear pressure-velocity relationship,such as in the velocity equation (10) outlined above, yields goodresults. In that case the fitting process yields values for thecoefficients a and b.

In an alternative embodiment the cuff measurement and velocity detectionand perturbation can all be made on a common limb, such as a single arm.Since the process of making a cuff measurement involves occlusion of thelimb, measurements of perturbation velocity during cuff pressurizationyield results different to those in an unperturbed limb. In oneembodiment, measurements of velocity would be made before or after cuffinflation and the measured velocities and pressures would thus besomewhat offset in time. In a patient with stable blood pressure thismay not introduce significant errors, although a typical cuff inflationtime of 30-45 seconds implies time offsets of that order of magnitudebetween the velocity and pressure measurements. In cases where this timeoffset introduces unacceptable errors, the determination technique canbe modified to introduce some averaging or trending. As an example,alternating velocity and cuff blood pressure measurements could be madeover a period of minutes and the results could be averaged to reduceerrors due to blood pressure fluctuation with time. Other forms of timeseries manipulation familiar to one skilled in the art could be used todevelop a valid relationship between blood pressure and exciter waveformvelocity using the pairs of velocity and pressure measurements obtainedby this technique.

In a further embodiment of the pressure-velocity relationshipdetermination process, an advantage can be taken of the fact thatmeasurement of blood pressure by an occlusive cuff measurement involvesa controlled and known modification of the transmural pressure in theartery. In this embodiment, depicted in FIG. 12, an occlusive cuff 811is placed over the exciter 202 and noninvasive sensor 210. The occlusivecuff 811 can also serve the function of cuff 110 in FIG. 1. The pressurein cuff 811 is controlled via tube 812 connected to processor 100. Theexciter 202 and sensor 210 separation and cuff size are chosen so thatthe exciter 202, the detector 210 and the portion of the limb betweenthem are all covered by the cuff 811.

As the pressure in the cuff 811 is increased, the transmural pressure inthe artery is decreased by a known amount. Thus, during the period ofcuff 811 inflation and deflation a variety of transmural pressures areexperienced in the artery and a variety of velocities will be observed.Since the cuff 811 pressure is known at all times, and the end point ofthe cuff measurement is measurement of systolic, diastolic and meanpressure measurement, it is possible after the measurement toreconstruct the value of transmural pressure at each point in timeduring the occlusive cuff measurement. This time series of varyingtransmural pressures can then be regressed against the time series ofvelocities measured over the same time interval to produce asophisticated and highly accurate determination of the velocity pressurerelationship over a range of transmural pressures from zero to systolicpressure. Increased accuracy and robustness and insensitivity to patienttemporal pressure fluctuation can clearly be obtained by repetition ofthis determination and use of averaging or other time series processingto minimize errors due to measurement inaccuracy and to patient pressurefluctuation.

While the velocity equation (10) is commonly adequate, in some instancesmore complex functions are appropriate to describe the pressure-velocityrelationship and functional forms such as quadratic, cubic or other morecomplex analytical functions can be used. In such cases the followingimprovement to the above embodiments can be important.

In each of the pressure-velocity determination embodiments describedabove, only the pressure values of mean, systolic and diastolic pressureare measured. The following improvement can be applied to each of them.The noninvasive sensor signal is filtered to provide an outputrepresentative of the naturally occurring blood pressure pulse as afunction of time during a given cardiac cycle. Similarly, the velocityof the exciter waveform is determined as a function of time during agiven cardiac cycle. By using the values of mean and diastolic andsystolic pressure determined by the calibration (e.g. occlusive cuff)measurement to scale a naturally occurring blood pressure waveformmeasured contemporaneously with the cuff measurement by the noninvasivesensor, a calibrated pressure waveform is determined for the bloodpressure. While sensor movement and a variety of other phenomena limitthe use of this calibrated waveform to a relatively short period oftime, it can be used during any of the pressure-velocity relationshipdetermination procedures described above to yield many pressure-velocitymeasurement pairs during a single cardiac cycle. The use of these extrapoints may improve the accuracy of the relationship determination,particularly in circumstances where the relationship functionality ismore complex than can be described by a linear relationship, such as anonlinear relationship.

In each of the embodiments described above, an occlusive cuff is used toocclude blood flow in an artery of the patient. Other occlusive devicessuch as bladders, bands, pressurized plates or diaphragms can be usedwith equal effect.

In each of the embodiments described above, determination of thepressure-velocity relationship is made from a series ofpressure-velocity pair measurements made over a range of pressures. Ingeneral, it is possible to extrapolate this relationship outside therange of the measurements used in the determination. The range overwhich such extrapolation is valid is determined based on examination ofdata from a study of multiple patients, and is related to the form ofthe pressure-velocity relationship and the values of its coefficients.The decision process embodied in step 734 in FIGS. 6, 9 and 10, includesan analysis of whether such extrapolation can be extended from theregime of initial calibration to that of the presently measuredvelocity. If not, the calibration process of step 750 is initiated andthe determination process described in this section is repeated.

A variety of other factors are considered in making the determination asto whether a recalibration, step 750, is required. These factors includeexamination of the amplitude and phase of the exciter waveform and anexamination of their dependence on frequency, detector-exciterseparation, and on various other factors including each other.

REDETERMINATION OF THE PRESSURE-VELOCITY RELATIONSHIP

Subsequent to the initial determination of the pressure-velocityrelationship described above, it is desirable to periodically determinewhether that relationship is still applicable. The relationship maybecome less applicable with time because of physiological changes in thepatient due to endogenous or exogenous chemicals in the body that canaffect the arterial muscular tone and, thus, the velocity of propagationof the exciter waveform. Even in the absence of changes in the patient,imperfect determination of the relationship due to measurement errorsmay lead to the need to check or redetermine the relationshipperiodically during a monitoring procedure.

The determination procedures described above involve the use of anocclusive cuff. While these determination procedures can be repeatedperiodically, there is a limit to the frequency of such measurements dueto the fact that each measurement results in a period on the order of aminute in which the circulation of the limb is impaired. Furthermore,occlusive cuff measurement is uncomfortable and therefore it isdesirable to minimize its use. Accordingly it is desirable for there tobe a technique of redetermining the velocity pressure relationship whichdoes not involve a conventional occlusive cuff measurement and which isrelatively comfortable and pain free, which is rapid compared to anocclusive cuff measurement and which can be repeated frequently.

In FIGS. 9 and 10, this process is represented by step 901 in which thepressure-velocity relationship is periodically redetermined. Theinterval of such redetermination is affected by the frequency ofexpected changes in the relationship. This is expected to be relativelyslow on the scale of the cardiac cycle and should probably be chosen tobe long with respect to the respiratory cycle to avoid interference.Time constants of the order of τ=30 seconds or more are suitable, butother time constants may also be appropriate. Subsequent to eachredetermination, the previously determined historical relationship iscompared with the new relationship in step 902. If the relationship haschanged significantly, the relationship used in the determination ofpressure is updated in step 903. As part of this process, averaging ofthe variously redetermined historical relationships or other time seriesanalysis may be used to provide increasingly accurate relationships foruse as the time elapsed since the initial calibration increases.

In the embodiment of redetermination described here, a relationship ofthe type of the velocity equation (10) is assumed. This technique can begeneralized to other functional forms of the relationship. In thefunctional form of the velocity equation (10), it is necessary todetermine the constants a and b corresponding to the offset and sloperespectively, of the relationship. In this embodiment, two separateoperations are used to determine the two coefficients a and b.

To determine the relationship slope b, the embodiment depicted in FIG.12 is used. The pressure in cuff 811 is varied in accordance with a timedependent pressure function δP(t). The function δP(t) typically has theform of a square wave of amplitude 10 mm Hg, period 30-60 seconds, andmean pressure of 5 mm Hg. However, alternate functional forms such assinusoids, triangular waves and other shapes can also be used, andlarger and smaller amplitudes and offset pressures can also be used. Inthe example described here, the artery is subject to alternatingpressures of 0 mm Hg and of 10 mm Hg. For constant diastolic andsystolic pressures, the transmural pressures at the diastolic andsystolic points, thus, alternate between (P_(D), P_(S)) and (P_(D) --10,P_(S) --10). The corresponding measured velocities are therefore(Vel(P_(D)), Vel(P_(S))), and (Vel(P_(D) --10), Vel(P_(S) --10)). Thecoefficient b can be determined using the formula:

    b=(Vel(P.sub.S)-Vel(P.sub.S --10))/10=(Vel(P.sub.D)-Vel(P.sub.D --10))/10 (11)

Clearly, averaging over longer periods than the time constant of asingle period of δP(t) leads to increased accuracy of this measurement.

In one embodiment, the above technique for redetermination can be usedalone as a determinant of the need for the calibration step of step 750in FIGS. 6, 9 and 10 to be repeated. In an alternative embodiment,continual updating of the value of b allows continual determination ofthe value of pressure without the need for a recalibration. As anillustration, the equation:

    P.sub.D (Pred)=P.sub.D0 +(Vel.sub.D -Vel.sub.D0)/b         (12)

can be used at any time to predict diastolic pressure if the value of bhas remained unchanged since the initial calibration. In the case that ais relatively constant, and that b has changed but has been continuouslymonitored, the prior equation can be replaced by the equation: ##EQU5##

In a further embodiment of the recalibration process, the coefficient acan also be periodically redetermined. There are a number of ways todetermine the offset a. In a preferred embodiment, the cuff 811 in FIG.12, is rapidly inflated to a pressure between the diastolic and systolicpressures of the last few pulses. At the time in the cardiac cycle inwhich cuff pressure equals or is within some determinable increment ofintraarterial pressure, the artery will close or reopen depending on thephase of the cardiac cycle. Many signatures can be observed of thisarterial closing or opening. These include Korotkoff sounds, wallmotion, arterial volume monitoring, plethysmography, blood flow andelectrical impedance.

The time in the cardiac cycle at which a signature appears can then becorrelated with the cuff pressure in cuff 811, and the waveshape of thevelocity pulses of nearby cardiac cycles can be used to associate asingle velocity with a single pressure (Vel1, P1). From this pair, thevalue of coefficient a can be calculated using the formula Vel1=a+bP1.While this measurement of coefficient a involves application of amoderate pressure to cuff 811, the pressure is less than the occlusivepressure associated with a conventional blood pressure cuff measurement.Furthermore, the pressure need only be applied for the duration of oneor at most several cardiac cycles. This is in contrast to a conventionalcuff measurement in which the cuff must be fully or partially inflatedover a significant number of cycles, typically of the order of 30-60seconds. This instantaneous single value measurement can thus be mademore rapidly and less traumatically than a multi-valued conventionalocclusive cuff pressure measurement.

MULTIPLE PERTURBATIONS

For each of the different embodiments described hereto, an additionalembodiment is described using multiple perturbation waveforms. All thefeatures and advantages of the prior embodiments are applicable to theseembodiments.

In the case of each of the previously described embodiments anembodiment is described in which the apparatus further induces a secondexciter waveform into the arterial blood. An example second exciterwaveform is one that has a frequency different from that of the firstexciter waveform. It is noted that although the discussion of the secondembodiment concentrates on a second exciter wave, any number of two ormore exciter waves can be used to determine the perturbation velocitymeasurement.

In operation, processor 100 generates two exciter waveforms andcommunicates the waveforms to the exciter 202 via air tube 107. Theexciter 202 responds by inducing both exciter waveforms into thepatient. Noninvasive sensor 210 generates a signal responsive to ahemoparameter and transmits the signal to the processor 100 via wire109.

The processor filters the noninvasive sensor signal into components ofthe natural waveform, a first exciter waveform, a second exciterwaveform and noise. The processor determine the phase relationship ofthe first exciter waveform to a first reference input and determines thephase relationship of the second exciter waveform to a second referenceinput.

Once the processor has determined the phase of the exciter waveforms,the processor then generates a plurality of points, the slope of whichrelates to the velocity of the exciter waveform. This is shown in FIG.8c, where the slope of the line is 2πd/Vel, and where d is distance andVel is velocity. Since the distance is fixed and the slope is related toblood pressure, and since the slope changes based on changes in bloodpressure, the velocity of the exciter waveform is determined.

The technique described above yields a measurement of the groupvelocity. In contrast, the techniques described in previous embodimentsresult in the measurement of a phase velocity or of a pseudo-phasevelocity in the case that the value of n of the phase equation (7) cannot be uniquely determined. In a dispersive system these values need notalways agree. However, phase, group and pseudo-velocity aremonotonically varying functions of pressure. Thus, a measurement of anyone of the three is a basis for a pressure prediction, so long as theappropriate pressure-velocity relationship is used.

An additional benefit of the use of multiple frequency perturbations isthat it allows the unique determination of the value of n in the phasemeasurement equation described above. The unique determination of thevalue of n is also called resolving the cycle-number ambiguity. Thisallows the use of actual phase velocity, rather than of thepseudo-velocity described earlier in the multi-perturbation analogues ofthe embodiments depicted in FIGS. 6, 9 and 10.

Once the velocity is determined, a prediction of blood pressure is madeaccording to FIG. 8a, showing the relationship of velocity to pressure.Thus, it is possible to determine the blood pressure with few, or zero,calibrations.

Another embodiment is depicted in FIG. 11 showing a cross section of anexciter 202 and noninvasive sensor 210 at the same position above theblood vessel 220. The proximate location of the exciter and the sensorpermits measurement of the blood vessel's response to the perturbations.In this embodiment, the noninvasive sensor is responsive to ahemoparameter such as blood flow or blood volume. These parameters canbe measured with a sensor such as a photoplethysmograph. Detectedchanges in the blood vessel due to the natural pulsatile pressure arecalibrated using external exciter pressure oscillations and comparedagainst the sensor signal by the processor.

DETERMINATION OF THE PATIENT'S PHYSICAL CONDITION

A. In the parent patent applications of Caro et al, and discussed above,techniques are described for the measurement of physiological parameterssuch as blood pressure, arterial elasticity, cardiac output, and otherparameters. As part of those techniques, a procedure is described todetermine a relationship between a patient's blood pressure P and thevelocity of propagation Vel of an induced high frequency pressureperturbation along the artery. This relationship can have various formsand can be described generally as:

    Vel=f(P)                                                   (14)

where f(P) is a function of pressure. An important step in the Carotechniques is the determination of the nature of the function f over arange of pressure in a given patient. In the Caro techniques, therelationship of equation (14) was used to determine pressure following ameasurement of velocity Vel. In the present embodiments, we describe theuse of the information contained in the form of the function f todetermine information relating to the physical condition of the arterialsystem.

In this context, we define several terms. As described, the propagationof the exciter waveform is a function of several factors. The waveformpropagation is a function of the blood pressure in the artery and afunction of the artery's mechanical properties. For example, as theblood pressure increases the velocity increases, and stiffer arteriesalso tend to be associated with increased velocity. Therefore, as usedherein propagation parameters include parameters such as velocity,pseudo-velocity, amplitude and attenuation and their respectivecomponents such as offset and slope. The general mechanical factors thatinfluence propagation of the exciter waveform are defined as mechanicalproperties such as arterial compliance, arterial distensibility andarterial elasticity.

The information regarding the physical condition of the arterial systemcan then be used to generate an output that can be used by health carepersonnel in the diagnosis and management of diseases such ascardiovascular disease.

B. In the previously referenced patent applications by Caro et al, anddiscussed above, there is described an invention capable of determiningthe relationship between the velocity of propagation of a pressure wavein a human arterial segment and the pressure in that segment. In theformalism of this application, the previously discussed inventiondescribes a technique for noninvasively determining the nature of therelationship described by equation (14) and of determining the natureand detailed description of the function f(P).

Now from theoretical considerations, we expect that the velocity ofpropagation of a pressure wave in the arterial system can be modeled bythe Moens-Korteweg equation (Moens, A.I.: Die Pulskurve. Leiden: E.J.Brill.; 1878.):

    Vel= (Eh)/(2pr)!.sup.1/2                                   (15)

or by the Bramwell-Hill equation (Bramwell, J. C., and Hill, A. V.:Proc. R. Soc. Lond. Biol.!:93:298, (1922).), which is substantiallyidentical for the case of a thin-walled tube:

    Vel= r/(2ρ(dr/dP))!.sup.1/2  1/(2ρD)!.sup.1/2      (16)

where r is the vessel radius, ρ is the blood density, E is the elasticmodulus of the artery wall, h is the wall thickness, (dr/dP) is thevessel compliance, D is the vessel distensibility, and P is the arterialpressure. Thus by comparing the experimentally determined function f(P)of equation (14) with the equations of (15) and (16), information can beobtained about the physical properties of the arterial system E, r, h, Dand ρ and their dependence on pressure.

Equations 15 and 16 are relatively simple relationships and are providedas examples. More complex and more accurate relationships can bedeveloped by theory or by experiment and may include models using neuralnetworks and patient data. Additionally, some more complex relationshipshave been reported in the literature and are known by those skilled inthe art. R. Holenstein, A Viscoelastic Model for use in PredictingArterial Pulse Waves, J. Biom. Eng., vol. 102, p. 318 (1980). Thediscussion that follows can clearly be generalized to include bothrelatively simple models and more complex models.

Most human arteries demonstrate a nonlinear relationship between radiusand pressure. A typical radius vs. pressure curve for the radial arterywill be similar to that shown in FIG. 13, where we have plotted anequation which has been successfully used to model radius vs. pressuredata from in vivo experiments on the radial artery. Y. Tardy,Non-invasive Estimate of the Mechanical Properties of PeripheralArteries from Ultrasonic and Photoplethsmographic Measurements, Clin.Phys. Physiol. Meas., vol. 12, no. 1, p. 39 (1991).

Because the arterial compliance (dr/dP) decreases monotonically withincreasing arterial pressure, the pressure wave velocity increasesmonotonically with increasing arterial pressure. See, for example, J.Pruett, Measurement of Pulse-Wave Velocity Using a Beat-SamplingTechnique, Annals Biomed. Eng., vol. 16, p. 341 (1988); and M. Landowne,A Method Using Induced Waves to Study Pressure Propagation in HumanArteries, Circulation Res., vol. 5, p. 594 (1957). This relationship isshown in FIG. 14.

C. Determine Arterial Distensibility

In one embodiment, the apparatus of the previously described inventions,discussed above, is used to determine the relationship of equation (14)and the function f(P). A value for the arterial distensibility, D(P), asa function of pressure is then determined from the equation:

    D(P)=1/ 2ρ(f(P)).sup.2 !                               (17)

where the value of ρ is either measured for that patient, or isgenerally assumed to be relatively invariant from patient to patient andequal to a value determined from an average over a pool of patients. Formany purposes it may be adequate to assume that the value of ρ is thatof water, or ρ=1, since the primary constituent of blood is water.

The values of distensibility as a function of pressure are then storedin memory and displayed for the use of health care personnel indiagnosis or treatment of the patients disease.

D. Determine Arterial Compliance

In another embodiment, the method and apparatus of the previousembodiment are combined with a device which has the capability of makinga measurement of the arterial radius r. An example of such a devicecould be an ultrasound imaging device or other ultrasound "pulseranging" device in which a pulse of ultrasound is directed at an angleacross the artery and a reflection is detected from both walls of theartery. By combining knowledge of the speeds of sound in tissue andblood with information concerning the angle with which the direction ofpropagation of the ultrasound beam intersects the arterial axis, ameasurement of the time elapsed between detection of the reflectionsfrom each of the arterial walls can be analyzed to provide a measurementof arterial diameter.

Techniques for determining arterial diameters, such as that describedabove, have been widely described in the scientific literature. Examplesof techniques for measuring arterial diameters are described in J.Meister, Non-invasive Method for the Assessment of Non-linear ElasticProperties and Stress in Forearm Arteries in Vivo, J. Hypertension, vol.10 (supp 6), p. 523 (1992); J. Levenson, Pulsed Doppler: Determinationof Diameter, Blood Flow, Velocity, and Volume Flow of Brachial Artery inMan, Cardiovascular Res., vol. 15, p. 164 (1981); and Y. Tardy,Non-invasive Estimate of the Mechanical Properties of PeripheralArteries from Ultrasonic and Photoplethsmogaphic Measurements, Clin.Phys. Physiol. Meas., vol. 12, no. 1, p. 39 (1991).

Other means for determining arterial diameter can include x-raymeasurement following injection of contrast media into the circulatorysystem, the use of Magnetic Resonance Imaging techniques, or the manualentry into the device of values measured by a separate machine.

Following determination of the value of radius r, the arterialcompliance C(P) or dr/dP can be determined according to the formula:

    C(P)=dr/dP=rD                                              (18)

F. In each of the above mechanical property determinations, we havedescribed determination of a property as a function of blood pressure.In a simplified embodiment, the mechanical property can be determined ata single blood pressure, such as the resting blood pressure of thepatient. For this embodiment, equation 17 becomes:

    D=1/ 2ρ(Vel).sup.2 !                                   (20)

for the distensibility, D, at a specific pressure for which the velocityof propagation is Vel. In such an embodiment, no calibration ordetermination of the relationship between velocity and pressure isrequired. Instead, health care decisions are made simply on the basis ofthe magnitude of the velocity in a patient at rest, or of one of thederived mechanical properties such as arterial compliance,distensibility or elasticity.

In a refinement of this embodiment, the patient's blood pressure ismeasured and the value of the mechanical property or the velocity iscompared with a previously stored value. The previously stored value canbe, for example, that of the patient stored from his last visit, or thatof a population of patients.

The resulting arterial compliance can then be displayed in display 120of FIG. 1.

E. Determine Arterial Elasticity

In another embodiment, the arterial wall thickness h can be determinedeither by a separate apparatus or by a subsystem incorporated within theabove described apparatus. An example of a technique for making thismeasurement is described in the work of Y. Tardy, Non-invasive Estimateof the Mechanical Properties of Peripheral Arteries from Ultrasonic andPhotoplethsmographic Measurements, Clin. Phys. Physiol. Meas., vol. 12,no. 1, p. 39 (1991); and Y. Tardy, Dynamic Non-invasive Measurements ofArterial Diameter and Wall Thickness, J. Hypertension, vol. 10 (supp 6),p. 105 (1992).

Following determination of r and h, the arterial elastic modulus E canbe determined from the equation

    E(P)= 2ρr(f(P)).sup.2 !/h                              (19)

Note that arterial thickness is a property that changes slowly over alengthy period of time, such as years, due to biological changes in thematerial of the arteries. This is in contrast to E and r which canchange over periods as short as minutes in response to endogenous orexogenous chemical stimulus.

Therefore in situations where a drug regimen is being optimized using Eas the parameter to be maximized or otherwise affected, where timesequences of measurements of arterial elasticity need to be made in agiven patient, measurement of relative elasticity may be adequate. Inthis case, no specific measurement need be made of absolute wallthickness.

A variety of derived parameters such as arterial wall stress or straincan be determined following the determination of the underlying physicalproperties of the artery. These parameters provide useful informationfor health care personnel. In the case where relative measurements onlyare required in a given patient, the parameter Q=(Eh) might profitablybe used.

F. In each of the above mechanical property determinations, we havedescribed determination of a property as a function of blood pressure.In a simplified embodiment, the mechanical property can be determined ata single blood pressure, such as the resting blood pressure of thepatient. For this embodiment, equation 17 becomes:

    D=1/ 2ρ(Vel).sup.2 !                                   (20)

for the distensibility, D, at a specific pressure for which the velocityof propagation is Vel. In such an embodiment, no calibration ordetermination of the relationship between velocity and pressure isrequired. Instead, health care decisions are made simply on the basis ofthe magnitude of the velocity in a patient at rest, or of one of thederived mechanical properties such as arterial compliance,distensibility or elasticity.

In a refinement of this embodiment, the patient's blood pressure ismeasured and the value of the mechanical property or the velocity iscompared with a previously stored value. The previously stored value canbe, for example, that of the patient stored from his last visit, or thatof a population of patients.

G. Other Derived Parameters

In another embodiment, the derivative of velocity with respect topressure, dv/dp, can be determined from the measured relationshipbetween velocity and pressure. Also, the derivative of amplitude withrespect to pressure and attenuation with respect to pressure can bedetermined. This, and other higher order derivatives of the functionf(P) can be expected to show a dependence of both magnitude and form onaging and on degree of cardiovascular disease.

H. Additional Embodiments to Determine A Physical Condition

In a refinement of each of the above embodiments, information is storedin the memory of the processor performing the determination of thevarious arterial properties from the measured function f(P). Thedetermined physical properties are then compared with that storedinformation and 'information about the state of health of the patient isderived from a comparison of the measured values of physical propertiesof the artery and the stored information. As an example, it is likelythat excessively stiffened arteries are symptomatic of underlyingcardiovascular disease.

In one example, the processor contains information concerning thearterial distensibility or compliance or elastic modulus as a functionof age, weight and other patient characteristics. This information wouldbe in the form of average values of these properties for "healthy" and"diseased" patients, together with the standard deviations of thedistributions of values of these parameters determined from a study of across section of the population.

By comparison of the measured values of, say, elastic modulus with thestored data from the patient population studied, a determination couldbe made of the probability that the patient was "healthy" or "diseased"and this information could be displayed for use by health carepersonnel.

In a preferred technique, the measurements described in the embodimentsare performed at a variety of anatomical sites. Placement of the deviceover the radial artery and measurement of radial artery properties hasadvantages of anatomical convenience. However, numerous other sites canbe used and similar measurements can be performed in the brachial,carotid, tibial, illiac and femoral arteries as common examples. Venousmeasurements can also be performed and may be useful to determine, forexample, the return blood flow to the heart and any problems that theblood is having in returning to the heart.

In each of the above embodiments, the velocity of propagation of thepressure perturbation is measured and various arterial physicalproperties are derived therefrom. In addition, it is often possible tomeasure the attenuation of the propagating perturbation in the artery.This measurement can be conveniently made by making use of informationcontained in the received exciter waveform signal at several separatefrequencies or distances.

The attenuation coefficient of propagation also is dependent on avariety of important physical properties of the artery. Thus,measurement of the attenuation coefficient will allow the deduction ofthe values of various additional arterial properties. As an example, theattenuation in the artery is a function of wall viscosity and can bedescribed by a variety of simple models such as those in M. Anliker,Dispersion and Attenuation of Small Artificial Pressure Waves in theCanine Aorta, Circulation Res., vol. 23, p. 539 (October 1968).Measurement of attenuation may thus yield the derived quantity of wallviscosity of the arterial segment under consideration. The frequencydependence of attenuation can also be measured and is also clinicallyuseful.

I. Clinical Research to Support Utility of the Embodiments

The embodiment depicted in FIG. 1 was used to continuously monitor thepropagation velocity of a pressure perturbation in the 100-200 Hz rangein the radial artery. A transmitted exciter waveform (excitation signal)was delivered to the radial artery just distal to the elbow by theexciter 202 (pneumatic bladder). A noninvasive sensor 210 (piezoelectrictransducer) was placed over the radial artery proximal to the wrist todetect both the received exciter waveform (propagated signal) and thephysiological waveform (arterial pulse). Arterial pressure was measuredsimultaneously in the contralateral arm by an arterial catheter. Thetransmitted exciter waveform, received exciter waveform, and arterialblood pressure were continuously sampled and stored by a computer foranalysis. The velocity of the exciter waveform was determined from thepropagation distance and the difference in phase between the transmittedexciter waveform and the received exciter waveform.

The continuous nature of a single frequency sinusoidal excitation signalprevents a unique determination of the total propagation phase, becausethe number of wavelengths between the exciter position and the sensorposition is unknown. In this clinical study information from multiplefrequencies was used, together with the assumption that the variation ofvelocity with frequency is small, to determine the total phase and,hence, the velocity. This assumption is supported by M. Anliker,Dispersion and Attenuation of Small Artificial Pressure Waves in theCanine Aorta, Circulation Res., vol. 23, p. 539 (October 1968).

Twenty patients undergoing major elective surgery requiringintraarterial blood pressure monitoring were studied. The results ofthis study was reported in R. Pearl et al., Anesth. 83,A481 (1995).

VARIATIONS ON THE DISCLOSED EMBODIMENTS

Additional embodiments include an embodiment in which two or moredetectors are positioned along the artery at different distances from asingle exciter, and an embodiment in which two or more exciters arepositioned along the artery at different distances from one or moredetectors. In each of these embodiments, the information obtained fromeach exciter detector pair can be analyzed independently. The multiplyredundant measurements of pressure that result can be combined toprovide a single pressure determination that may be both more accurateand more immune from noise, motion artifact and other potential errorsources. Similar redundancy can be achieved in the embodiments that usemultiple exciter waveforms' by analyzing the results at each frequencyindependently and combining the results to provide enhanced robustness.

In addition, any combination of more than two elements (e.g. twoexciters and one detector, two detectors and one exciter, one exciterand three detectors) allows the value of n in the phase equation (7) tobe uniquely determined so long as the spacing of two of the elements issufficiently small to be less than a wavelength of the propagatingperturbation. Since the possible range of perturbation wavelengths at agiven pressure can be determined from a pool of patients, selection ofthe appropriate spacing is straightforward and can be incorporated intothe geometrical design of the device.

CONCLUSION

A close relationship between physiological parameters and hemoparameterssupplies valuable information used in the present invention. Theperturbation of body tissue and sensing the perturbation also suppliesvaluable information used in the present invention. Although thepreferred embodiment concentrates on blood pressure, the presentinvention can also be used to analyze and track other physiologicalparameters such as vascular wall compliance, changes in the strength ofventricular contractions, changes in vascular resistance, changes influid volume, changes in cardiac output, myocardial contractility andother related parameters.

Calibration signals for the present invention can be obtained from avariety of sources including a catheter, manual determination, or othersimilar method.

The DC offset for the physiological parameter waveform can be obtainedin a variety of ways for use with the present invention.

The exciter of the preferred embodiment uses air, but any suitable fluidcan be used. Moreover, various exciter techniques can be used forinducing an exciter waveform into the patient such as an acousticexciter, an electromagnetic exciter and an electromechanical exciter(e.g. piezoelectric device).

Various noninvasive sensors have been developed for sensinghemoparameters. These sensor types include piezoelectric,piezoresistive, impedance plethysmograph, photoplethysmograph, varioustypes of strain gages, air cuffs, tonometry, conductivity, resistivityand other devices. The present invention can use any sensor thatprovides a waveform related to the hemoparameter of interest.

Having disclosed exemplary embodiments and the best mode, modificationsand variations may be made to the disclosed embodiments while remainingwithin the subject and spirit of the invention as defined by thefollowing claims.

What is claimed is:
 1. A monitor for use in determining a patient'sphysical condition, comprising:an exciter adapted to be positioned overa blood vessel of the patient and configured to induce a transmittedexciter waveform into the patient; a noninvasive sensor spaced apartfrom said exciter, adapted to be positioned over said blood vessel andconfigured to sense a hemoparameter and to generate a noninvasive sensorsignal representative of said hemoparameter containing a component of areceived exciter waveform; and a processor coupled to said noninvasivesensor and configured to receive said noninvasive sensor signal and toprocess said noninvasive sensor signal to determine a physical propertyof the patient in order to provide a condition signal related to saidphysical condition.
 2. The monitor of claim 1, further comprising:adevice configured to provide a pressure signal representative of a bloodpressure of the patient; and wherein said processor is configured toreceive said pressure signal and to provide said pressure signal as anoutput.
 3. The monitor of claim 2, wherein:said processor is configuredto compare said condition signal to a previously stored condition signalto provide a relative condition signal related to said physicalcondition.
 4. The monitor of claim 1, further comprising:a deviceconfigured to provide a pressure signal representative of a bloodpressure of the patient; and wherein said processor is configured toreceive said pressure signal and to process said pressure signal toprovide said condition signal.
 5. The monitor of claim 4, wherein:saidprocessor is configured to compare said condition signal to a previouslystored condition signal to provide a relative condition signal relatedto said physical condition.
 6. The monitor of claim 4, wherein:saidprocessor is configured to determine a propagation parameter of saidreceived exciter waveform and to process said propagation parameter andsaid pressure signal to provide said condition signal.
 7. The monitor ofclaim 6, wherein:said processor is configured to determine arelationship between said propagation parameter and said pressuresignal; and said processor is configured to provide said conditionsignal based at least in part on said relationship.
 8. The monitor ofclaim 4, wherein:said processor is configured to determine a derivativeof a propagation parameter of said received exciter waveform and toprocess said derivative and said pressure signal to provide saidcondition signal.
 9. The monitor of claim 8, wherein:said processor isconfigured to compare said condition signal to a previously storedcondition signal provide a relative condition signal related to saidphysical condition.
 10. The monitor of claim 4, wherein:said physicalcondition is a mechanical property of said blood vessel; and saidprocessor is configured to process said noninvasive sensor signal andsaid pressure signal to provide said condition signal related to saidmechanical property.
 11. The monitor of claim 10, wherein:said processoris configured to compare said condition signal to a previously storedcondition signal provide a relative condition signal related to saidphysical condition.
 12. The monitor of claim 1, wherein:said processoris configured to compare said condition signal to a previously storedcondition signal to provide a relative condition signal related to saidphysical condition.
 13. The monitor of claim 1, wherein:said processoris configured to determine a propagation parameter of said receivedexciter waveform and to process said propagation parameter to providesaid condition signal.
 14. The monitor of claim 1, wherein:saidprocessor is configured to store a relationship between a propagationparameter and a blood pressure; and said processor is configured toprovide said condition signal based at least in part on saidrelationship.
 15. The monitor of claim 1, wherein:said processor isconfigured to determine a derivative of a propagation parameter of saidreceived exciter waveform and to process said derivative to provide saidcondition signal.
 16. The monitor of claim 15, wherein:said processoris, configured to compare said condition signal to a previously storedcondition signal provide a relative condition signal related to saidphysical condition.
 17. The monitor of claim 1, wherein:said physicalcondition is a mechanical property of said blood vessel; and saidprocessor is configured to process said noninvasive sensor signal toprovide said condition signal related to said mechanical property. 18.The monitor of claim 17, wherein:said processor is configured to comparesaid condition signal to a previously stored condition signal provide arelative condition signal related to said physical condition.
 19. Aprocessor for use in determining a patient's physical condition with anapparatus having an exciter positioned over a blood vessel of thepatient and configured to induce a transmitted exciter waveform into thepatient, and a noninvasive sensor spaced apart from said exciter andadapted to be positioned over said blood vessel and configured to sensea hemoparameter and to generate a noninvasive sensor signalrepresentative of said hemoparameter, said processor comprising:a firstinput configured to receive said noninvasive sensor signal; and adetermination routine configured to process said noninvasive sensorsignal to determine a physical property of the patient in order toprovide a condition signal related to said physical condition.
 20. Theprocessor of claim 19, wherein said apparatus includes a deviceconfigured to provide a pressure signal representative of a bloodpressure of the patient, said processor further comprising:a secondinput configured to receive said pressure signal; and an output pressureroutine configured to provide said pressure signal as an output.
 21. Theprocessor of claim 20, further comprising:a comparison routineconfigured to compare said condition signal to a previously storedcondition signal to provide a relative condition signal related to saidphysical condition.
 22. The processor of claim 19, wherein saidapparatus includes a device configured to provide a pressure signalrepresentative of a blood pressure of the patient, said processorfurther comprising:a second input configured to receive said pressuresignal; and wherein said determination routine is configured to processsaid pressure signal to provide said condition signal.
 23. The processorof claim 22, further comprising:a comparison routine configured tocompare said condition signal to a previously stored condition signal toprovide a relative condition signal related to said physical condition.24. The processor of claim 22, further comprising:a propagation routineconfigured to determine a propagation parameter of said received exciterwaveform; and wherein said determination routine is configured toprocess said propagation parameter and said pressure signal to providesaid condition signal.
 25. The processor of claim 24, furthercomprising:a relationship routine configured to determine a relationshipbetween a propagation parameter and blood pressure; and wherein saiddetermination routine is configured to provide said condition signalbased at least in part on said relationship.
 26. The processor of claim22, further comprising:a derivative routine configured to determine aderivative of a propagation parameter of said received exciter waveform;and wherein said determination routine is configured to process saidderivative and said pressure signal to provide said condition signal.27. The processor of claim 26, further comprising:a comparison routineconfigured to compare said condition signal to a previously storedcondition signal to provide a relative condition signal related to saidphysical condition.
 28. The monitor of claim 22, wherein:said physicalcondition is a mechanical property of said blood vessel; and saiddetermination routine is configured to process said pressure signal toprovide said condition signal related to said mechanical property. 29.The monitor of claim 28, further comprising:a comparison routineconfigured to compare said condition signal to a previously storedcondition signal to provide a relative condition signal related to saidphysical condition.
 30. The processor of claim 19, further comprising:acomparison routine configured to compare said condition signal to apreviously stored condition signal to provide a relative conditionsignal related to said physical condition.
 31. The processor of claim19, further comprising:a propagation routine configured to determine apropagation parameter of said received exciter waveform; and whereinsaid determination routine is configured to process said propagationparameter to provide said condition signal.
 32. The processor of claim19, further comprising:a memory configured to store a relationshipbetween a propagation parameter and blood pressure; and wherein saiddetermination routine is configured to provide said condition signalbased at least in part on said relationship.
 33. The processor of claim19, further comprising:a derivative routine configured to determine aderivative of a propagation parameter of said received exciter waveform;and wherein said determination routine is configured to process saidderivative to provide said condition signal.
 34. The processor of claim33, further comprising:a comparison routine configured to compare saidcondition signal to a previously stored condition signal to provide arelative condition signal related to said physical condition.
 35. Theprocessor of claim 19, wherein:said physical condition is a mechanicalproperty of said blood vessel; and said determination routine isconfigured to provide said condition signal related to said mechanicalproperty.
 36. The monitor of claim 35, further comprising:a comparisonroutine configured to compare said condition signal to a previouslystored condition signal to provide a relative condition signal relatedto said physical condition.
 37. A method for use in determining apatient's physical condition, comprising the steps of:inducing atransmitted exciter waveform into the patient at a first location;noninvasively sensing a hemoparameter at a location spaced apart fromsaid first location and generating a noninvasive sensor signalrepresentative of said hemoparameter containing a component of areceived exciter waveform; processing said noninvasive sensor signal todetermine a physical property of the patient in order to provide acondition signal related to said physical condition.
 38. The method ofclaim 37, further comprising the step of:providing a calibration signalrepresentative of the patient's physiological parameter and storing thecalibration signal; and wherein said processing step includes a step ofproviding said pressure signal as an output.
 39. The method of claim 38,further comprising the step of:comparing said condition signal to apreviously stored condition signal to provide a relative conditionsignal related to said physical condition.
 40. The method of claim 37,further comprising the step of:providing a calibration signalrepresentative of the patient's physiological parameter and storing thecalibration signal; and wherein said processing step is performed byprocessing said pressure signal to provide said condition signal. 41.The method of claim 40, further comprising the step of:comparing saidcondition signal to a previously stored condition signal to provide arelative condition signal related to said physical condition.
 42. Themethod of claim 39, further comprising the step of:determining apropagation parameter of said received exciter waveform; and whereinsaid processing step is performed by processing said propagationparameter and said pressure signal to provide said condition signal. 43.The method of claim 42, further comprising the step of:determining arelationship between said propagation parameter and said pressuresignal; and wherein said processing step is performed to provide saidcondition signal based at least in part on said relationship.
 44. Themethod of claim 39, further comprising the step of:determining aderivative of a propagation parameter of said received exciter waveform;and wherein said processing step is performed by processing saidderivative and said pressure signal to provide said condition signal.45. The method of claim 44, further comprising the step of:comparingsaid condition signal to a previously stored condition signal to providea relative condition signal related to said physical condition.
 46. Themethod of claim 39, wherein:said physical condition is a mechanicalproperty of said blood vessel; and said processing step is performed toprocess said noninvasive sensor signal and said pressure signal toprovide said condition signal related to said mechanical property. 47.The method of claim 46, further comprising the step of:comparing saidcondition signal to a previously stored condition signal to provide arelative condition signal related to said physical condition.
 48. Themethod of claim 37, further comprising the step of:comparing saidcondition signal to a previously stored condition signal to provide arelative condition signal related to said physical condition.
 49. Themethod of claim 37, further comprising the step of:determining apropagation parameter of said received exciter waveform; and whereinsaid processing step is performed by processing said propagationparameter to provide said condition signal.
 50. The method of claim 37,further comprising the step of:storing a relationship between apropagation parameter and a blood pressure; and wherein said processingstep is performed to provide said condition signal based at least inpart on said relationship.
 51. The method of claim 37, furthercomprising the step of:determining a derivative of a propagationparameter of said received exciter waveform; and wherein said processingstep is performed by processing said derivative to provide saidcondition signal.
 52. The method of claim 51, further comprising thestep of:comparing said condition signal to a previously stored conditionsignal to provide a relative condition signal related to said physicalcondition.
 53. The method of claim 37, wherein:said physical conditionis a mechanical property of said blood vessel; and said processing stepis performed to process said noninvasive sensor signal to provide saidcondition signal related to said mechanical property.
 54. The method ofclaim 53, further comprising the step of:comparing said condition signalto a previously stored condition signal to provide a relative conditionsignal related to said physical condition.
 55. A monitor for use indetermining a patient's physical condition, comprising:a deviceconfigured to provide a pressure signal representative of a bloodpressure of the patient; an exciter adapted to be positioned over ablood vessel of the patient and configured to induce a transmittedexciter waveform into the patient; a noninvasive sensor spaced apartfrom said exciter and adapted to be positioned over said blood vesselfrom said exciter and configured to sense a hemoparameter and togenerate a noninvasive sensor signal representative of saidhemoparameter containing a component of a received exciter waveform; anda processor coupled to said noninvasive sensor and configured to receivesaid noninvasive sensor signal and said pressure signal and to processsaid noninvasive sensor signal and said pressure signal to determine aphysical property of the patient in order to provide a condition signalrelated to said physical condition.
 56. The monitor of claim 55,wherein:said processor is configured to determine a velocity of saidtransmitted exciter waveform and to process said velocity to providesaid condition signal.
 57. The monitor of claim 56, wherein:saidprocessor is configured to determine a relationship between the velocityof said transmitted exciter waveform and said pressure signal; and saidprocessor is configured to provide said condition signal based at leastin part on said relationship.